Our current emphasis in the study of play is investigation of the motivational or other conditions promoting the appearance of play in the squirrel monkey. Attempts to extend to a nonhuman primate the demonstration by other laboratories of a modest enhancement of rodent play by low doses of morphine (the only pharmacologic agent to demonstrate selective enhancement of mammalian play) were discouraging. The addition of data from 2 pairs of young males did not support the modestly hopeful results obtained from a single pair last year. We suspect that the subtle effects of low doses of morphine are lost in the highly variable play behavior characteristic of primates. While each pair of animals tested played robustly during vehicle as well as morphine trials, differences among pairs greatly exceeded differences between experimental conditions. The sample size needed to overcome this problem is unrealistic for several reasons. We have therefore decided to discontinue morphine administration and are instead analyzing the baseline and vehicle data already collected, to assess the extent of pairwise differences in play. We expect these data to reveal considerable flexibility in the accommodations monkeys make in order to achieve a high level of play activity. We also plan to investigate more directly the relationship between stress and play, utilizing cortisol as a measure of stress. Most of the animal subjects and facilities already in place will be used for this new study.